1. Field of the Invention
The present invention relates generally to active solid state devices, specifically to apparatus and method for making and using sensors with nanodimensional features that are responsive to molecular compounds, organisms or gas molecules.
2. Description of Related Art
The use of nanowires and nanotubes for label-free direct real-time detection of biomolecule binding is known in the art. Nanowires and nanotubes have the potential for very high-sensitivity detection since the depletion or accumulation of charge carriers, which is caused by binding of a charged biological macromolecules at the surface, can affect the entire cross-sectional conduction pathway of these nanostructures. See, e.g., Direct Ultrasensitive Electrical Detection of DNA and DNA Sequence Variations Using Nanowire Nanosensors, by Jong-in Hahm and Charles M. Lieber, Nano Letters, 2004 (Vol. 4, No. 1 pp. 51-54), which is incorporated by reference (hereinafter Lieber). Lieber discloses measurable conductance changes associated with hybridization of a Peptide Nucleic Acid (PNA) receptor with complimentary Deoxyribose Nucleic Acid (DNA) target molecule. A practitioner skilled in the art will appreciate that a Peptide Nucleic Acid (PNA) receptor could be substituted with a Deoxyribose Nucleic Acid (DNA) receptor or a Ribose Nucleic Acid (RNA) receptor.
U.S. Pat. No. 7,301,199 discloses nanowires fabricated using laser catalytic growth (LCG), and is incorporated by reference in its entirety. In LCG, a nanoparticle catalyst is used during the growth of the nanoscale wire. Laser vaporization of a composite target composed of a desired material and a catalytic material creates a hot, dense vapor. The vapor condenses into liquid nanoclusters through collision with a buffer gas. Growth begins when the liquid nanoclusters become supersaturated with the desired phase and can continue as long as reactant is available. Growth terminates when the nanoscale wire passes out of the hot reaction zone or when the temperature is decreased. In LCG, vapor phase semiconductor reactants required for nanoscale wire growth may be produced by laser ablation of solid targets, vapor-phase molecular species, or the like. To create a single junction within a nanoscale wire, the addition of the first reactant may be stopped during growth, and then a second reactant may be introduced for the remainder of the synthesis. Repeated modulation of the reactants during growth is also contemplated, which may produce nanoscale wire superlattices. LCG also may require a nanocluster catalyst suitable for growth of the different superlattice components; for example, a gold nanocluster catalyst can be used in a wide-range of III-V and IV materials. Nearly monodisperse metal nanoclusters may be used to control the diameter, and, through growth time, the length of various semiconductor nanoscale wires. This method of fabricating nanowires is known in the art, and constitutes one method of creating nano-scale features.
The use of photolithography for fabrication of micron-scale features is well known in the art. In “standard” photolithography, multiple steps are performed to pattern features on a surface. In the initial step, the surface, which may be a p- or n-doped silicon wafer, is cleaned of surface contaminants. Persons skilled in the art will appreciate that many planar surfaces can be patterned in this way, including surfaces with multiple layers, such as a substrate of p- or n-doped silicon, a middle layer of insulating silicon dioxide (SiO2), with a top layer of metal. Next, adhesion promoters are added to the surface to assist in photoresist coating. Photoresist may be spin-coated onto the surface, forming a uniform thickness. The wafer containing the photoresist layer is then exposed to heat to drive off solvent present from the coating process. Next, a photomask, which may be made of glass with a chromium coating, is prepared. The features desired on the surface of the wafer are patterned on the photomask. The photomask is then carefully aligned with the wafer. The photomask is exposed to light, the transparent areas of the photomask allow light to transfer to the photoresist, the photoresist reacts to the light, and a latent image is created in the photoresist. The photoresist may be either positive or negative tone photoresist. If it is negative tone photoresist, it is photopolymerized where exposed and rendered insoluble to the developer solution. If it is positive tone photoresist, exposure decomposes a development inhibitor and developer solution only dissolves photoresist in the exposed areas. Simple organic solvents are sufficient to remove undeveloped photoresist. The techniques of “etch-back” and “lift-off” patterning are used at this stage. If the “etch-back” technique is used, the photoresist is deposited over the layer to be pattered, the photoresist is patterned, and the unpatterned areas of the layer are removed by etching. If the “lift-off” technique is used, photoresist is deposited followed by deposition of a thin film of desired material. After exposure, undeveloped photoresist is removed by the developer solvent and carries away the material above it into solution leaving behind the patterned features of the thin film on the surface. Removal of the remaining photoresist may be accomplished through oxygen plasma etching, sometimes called “ashing”, or by wet chemical means using a “piranha” (3:1 H2SO4:H2O2) solution.
Although widely used and extremely useful as a micron-scale patterning tool, “standard” photolithography is limited in the resolution of the features it can pattern. The ability to project a clear image of a small feature onto the wafer is limited by the wavelength of the light that is used, and the ability of the reduction lens system to capture enough diffraction orders from the illuminated mask. The minimum feature size that a projection system can print is given approximately by: CD=k1*(λ/NA); where CD is the minimum feature size (also called the critical dimension, target design rule); k1 (commonly called k1 factor) is a coefficient that encapsulates process-related factors, and typically equals 0.4 for production; λ is the wavelength of light used; and NA is the numerical aperture of the lens as seen from the wafer. According to this equation, minimum feature sizes can be decreased by decreasing the wavelength, and increasing the numerical aperture, i.e. making lenses larger and bringing them closer to the wafer. However, this design method runs into a competing constraint. In modern systems, the depth of focus (DF) is also a concern: DF=k2*(λ/(NA)2). Here, k2 is another process-related coefficient. The depth of focus restricts the thickness of the photoresist and the depth of the topography on the wafer. One solution known in the art is utilization of light sources with shorter wavelengths (λ), and creation of lenses with higher numeric apertures (NA). The drawback to this solution is the increasingly prohibitive high cost of fabricating complex sources and optics.
Nanoimprint Lithography (NIL) solves the problem of limited minimum feature sizes and high cost by patterning nano-scale features into a quartz plate, referred to as the “template” that can be applied directly to the surface of a wafer and transferring the pattern 1:1 into a photoresist layer. “Step and Flash Imprint Lithography,” by Resnick, D., et al., Solid State Technology, (2007), February, 39, which is incorporated in its entirety by reference, discloses the method to pattern nano-scale features by first imprinting the features into a photoresist layer and dry etching the imprint layer into the desired thin film layer on a wafer. The S-FIL process, now generally known in the art as Nanoimprint Lithography (NIL), requires that electron beam lithography be first used to “write” the desired imprint pattern into the template. The template may be a quartz plate substrate coated with a chromium (Cr) layer. The electron beam resist is patterned and the pattern is transferred into the Cr layer and the final three-dimensional relief structure is etched into the quartz plate or “template.” After transfer of the pattern into the quartz layer, the Cr layer is stripped, leaving an optically transparent template with the imprint pattern etched onto one surface.
To create the imprint pattern into a thin film layer on a wafer substrate, a low-viscosity photocurable monomer—known as the etch barrier—is dispensed on its surface. The transparent template is brought into contact with the monomer at a slight angle, creating a monomer wavefront that spreads across the surface and fills the three dimensional relief structures of the transparent template. UV light photopolymerizes the monomer and the template is separated from the wafer, leaving a solid replica of the reverse of the template on the substrate surface. Post-processing consists of a breakthrough etch of the residual layer of the monomer, followed by a selective etch into an organic layer and finally transfer of the pattern into the desired layer; for example a semiconductor thin film. Imprint lithography has been used to create feature CDs on the order of 20 nm in high density over large areas, e.g. 4-6″ wafers during a single imprint process.
In a similar fashion, the reverse process (S-FIL/R) can be accomplished. This is achieved by imprinting the surface using the template followed by spinning on an organic layer. The organic layer is etched back to expose the top surface of the silicon-containing imprint which is then selectively etched to the substrate using the organic layer as an etch stop. A final set of etching conditions is used to transfer the pattern into the substrate material. Nanoimprint Lithography has the advantage of being limited only by physical resolution of the template rather than being limited by wavelength and numeric aperture, as in standard photolithography. As new methods emerge for template fabrication, a corresponding increase in feature resolution can be expected.
U.S. Pat. No. 6,426,184 discloses a method for massively parallel synthesis of DNA, RNA, and PNA molecules utilizing photogenerated reagents (PGR), and is incorporated herein by reference. The method involves a microfluidic chamber comprising a series of wells that act as reaction sites with a transparent sealed cover. Within each well, a “linker” molecule functionalized with a “reactive group” is attached to the substrate. The reactive group couples a “spacer group” which then couples the first nucleotide to the surface. The nucleotide bears a “protection group” initial. The reactive precursor to the PGR is introduced through the microfluidic chamber into the well sites. Selective wells receive light using a spatial light modulating device during a given exposure step which results in a “photogenerated reagent” within each well that was exposed. PGR is activated only in the wells that are exposed to light, thereby causing a chemical reaction with the protection group, and “de-protecting” the terminal nucleotide in the nucleic acid sequence. The PGR is flushed from the system, and a select nucleotide with a “protection group” is introduced. The nucleotide with “protection group” is covalently bonded to the end of the nucleic acid sequence in the selected wells. In all other wells that do not get exposed to light, no reaction takes place and no nucleotide coupling occurs during that exposure cycle. After proper washing, oxidation, and capping steps, the addition of the cycle is repeated in such a fashion to synthesize any combination of nucleotides onto surface-anchored nucleic acid sequences that are specific to each well. The process is continued until the oligonucleotides of interest are constructed over the entire array. The chemistry of building oligonucleotides is well known in the art. Because the sequence is known for each well in the multiplex detection array, diagnostic tests that result in a signal transduction event can be performed by first identifying if a reaction occurs for a given well, and second by determining the position, and hence identity of the “known” anchor probe sequence.
“Light Directed Massively Parallel On-chip Synthesis of Peptide Arrays with t-Boc Chemistry,” by Gao, X., et al., Proteomics, (2003), 3, 2135 discloses PNA synthesis using t-Boc chemistry, and is incorporated by reference herein. This article is an example of chemical syntheses of anchor probe libraries known in the art.
What is needed is a cost-effective, time-efficient, reproducible method for fabricating arrays of nano-scale features on a single wafer to form a sensor device or a matrix of devices for multiplex detection of selected analytes using many simultaneous detection zones, by detecting changes in electrical characteristics of the nano-scale materials for each device. Method for making such sensors and arrays is needed.